Recent advances in our ability to reestablish flow to ischemic myocardium have raised hopes that salvage of jeopardized myocardium is now possible and may improve survival in afflicted patients. The reperfusion process itself, however, appears to involve its own injury. Evidence exists that a mechanism of this injury involves the oxidation of susceptible chemical moieties, such as the thiol groups of amino acids, thereby structurally changing important cellular proteins. H 2 0 2 will be targeted as the molecule most responsible for this oxidant stress. It is neutral, freely permeable, and is the nexus in the evolution of toxic oxygen metabolites during reperfusion. Creatine kinase (CK) is a critical enzyme in myocardial bioenergetics and is susceptible to this thiol oxidation. It is well recognized that reperfusion of ischemic myocardium is associated with a rapid accumulation of phosphocreatine (PCr) without a corresponding rise in adenosine triphosphate (ATP). In non-ischemic myocardium, this disequilibrium is prevented by CK activity which attenuates the fall in ATP by catalyzing the production of ATP from PCr. Using a multidisciplinary approach involving enzymatic assays, an ischemic/reperfused isolated heart model, and manipulation of H 2 0 2 during reperfusion, the effect of the reperfusion oxidant injury on myocardial CK activity, tissue bioenergetics, and recovery of mechanical function will be assessed. Specifically, the purpose of this study will be 1) to relate CK activity to H2O2 concentrations during reperfusion with recovery of mechanical function, CK activity, and tissue PCr/ATP concentrations. Expected is the elucidation of a mechanism for the alteration of high energy phosphate production during ischemia/reperfusion. An understanding of the biochemical mechanisms of critical enzyme inhibition during reperfusion can thus be related to myocardial bioenergetic imbalance with it subsequent mechanical dysfunctional sequelae. Elucidation of these mechanisms should provide insight into practical therapies.